High temperature cobalt-base sheet alloy

ABSTRACT

A SUPERIOR WROUGHT COBALT-BASE ALLOY FOR HIGH TEMPERATURE USE IS PROVIDED AT THE NOMINAL COMPOSITION, BY WEIGHT, OF 0.9 PERCENT CARBON, 25 PERCENT CHROMIUM 0.9 PERCENT TITANIUM, 1 PERCENT IRON, 15 PERCENT NICKEL, 7.7 PERCENT TUNGSTEN, 0.4 PERCENT ZIRCONIUM, 1 PERCENT COLUMBIUM, 2.7 PERCENT TANTATLUM, BALANCE ESSENTIALLY COBALT.

Jne 6, 1972 R.J.HEc|-11' ,EIAL

HIGH 'IEIPERA'IUREl COBALT-BASE SHEET* ALLOY 4 Sheets-Sheet 1' Filed Dec. 30, 1971 June-6, 1972 R. J. HECHT ETAL l 3,667,939

HIGH TEMPERATURE COBALT-BASE SHEET ALLOY "Filed Deo. 30, 1971 Ai Sheets-Sheet 2 l l I l l y; 46 4/5 f@ fz d? f6 a? June 6, 1972 R.J. HECHT ETAL 3,667,939

HIGH TEMPERATURE COBALT-BASE SHEET ALLOY Filed Dec. 30, 1971 4 Sheets-Sheet 5 Z Ma/f? a/aff' 055 Xga@ gaap

June 6, 1972 R. J. HEcHT ETAI- 3,567,939

HIGH TEMPERATURE COBALT-BASE SHEET ALLOY Filed Dec. 30, 1971 4 Sheets-Shea?l 4.

United States Patent O 3,667,939 HIGH TEMPERATURE COBALT-BASE SHEET ALLOY Ralph J. Hecht, West Palm Beach, and Richard J. Fenton, Palm Beach Shores, Fla., assignors to United Aircraft Corporation, East Hartford, Conn.

Filed Dec. 30, 1971, Ser. No. 102,747 Int. Cl. C22c 19/00 U.S. Cl. 75-171 3 Claims ABSTRACT OF THE DISCLOSURE A superior wrought cobalt-base allo'y for high temperature use is provided at the nominal composition, by Weight, of 0.9 percent carbon, 25 percent chromium 0.9 percent titanium, 1 percent iron, 15 percent nickel, 7.7 percent tungsten, 0.4 percent zirconium, 1 percent columbium, 2.7 percent tantalum, balance essentially cobalt.

The invention herein was made in the course of a contract with the Department of the Air Force.

BACKGROUND OF THE INVENTION The present invention relates to a high temperature, wrought cobalt-base alloy.

ln the gas turbine engine industry a need exists for a high temperature alloy, fabricable into sheet, of higher strength and thermal stability than those currently available on the market. lWhile a number of castable alloys are available, the variety of wrought alloys suitable for sheet applications is much more limited. Thus, many of the cobalt-base alloys, while possessed of the requisite high strengths and oxidation resistance for high temperature use are, in fact, not usable or fabricable in sheet form. Moreover, in addition to the usual criteria of strength and fabricability required in the high temperature sheet alloys, fatigue, creep and corrosion resistance must be provided as well. Furthermore, the alloys must be characterized by stability and excellent weldabilit'y for practical utilization.

There are a number of high temperature cobalt-base alloys known in the art as evidenced, for example, by the following patents: Wheaton 3,366,478; Wheaton 3,432,294; Rausch et al. 3,223,522; McQuillan et al. 3,314,784; and Thielemann et al. 3,333,957.

This body of art and, in fact, the experience of those skilled in the art, clearly demonstrates that this iield is one in which small but critical limitations are often determinative as to whether or not a given alloy is usable for the purpose intended. Nor is the art one of reasonable empirical certainty as the results of small changes in either the character or proportions of an alloy are not normally truly predictable.

'In the detailed description which follows, reference will be made to a number of contemporary sheet alloys which represent, in general, the best currently available on the market. These are as follows:

Designation Nominal Chemistry (by weight) Alloy A Ni, 20% Cr, 15% W, 1.5%

Mn, .08% C, bal. Co. Alloy B 22% Ni, 22% Cr, 14.5% W, 0.1%

C, 0.8% La, bal. Co. Alloy C 22% Cr, 1.5% Co, .1% C, 18.5%

Fe, 9% Mo, .6% W, bal. Ni. Alloy D 21.5% Cr, 9% Mo, 3.65%

(Cb-l-Ta), bal. Ni.

SUMMARY OF THE INVENTION The present invention contemplates as allow (WS-6) characterized by a medium grained microstructure staice bilized by a relatively high volume percent of carbides dispersed in a face-centered-cubic cobalt/chromium/ nickel/ tungsten solid solution matrix, particularly suitable for the fabrication and use in sheet form.

The nominal chemistry of the WS-6 alloy is, by weight, 0.9 percent carbon, 25 percent chromium, 0.9 percent titanium, l percent iron, l5 percent nickel, 7.7 percent tungsten, 0.4 percent zirconium, l per-cent columbium, 2.7 percent tantalum, balance essentially cobalt.

The particular preferred specification chemistry is as follows: 0183-097 percent carbon, 24-26 percent chromium, 0.8-1 percent titanium, 0.8-1.2 percent iron, 14- 16 percent nickel, 7.5-8 percent tungsten, 0.35-0.45 percent zirconium, 0.8-1.2 percent columbium, 2.5-3 percent tantalum, balance substantially cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS The as-cast WS-6 alloy in its preferred specification chemistry comprises about 17.5 volume percent MC-type carbides in a face centered cubic, cobalt-base Solid solution matrix. The analysis of the carbide and solid solution for the cast condition has been determined by electron microprobe analysis to be as follows:

Element Carbide Solid solution There appears to be a primary carbide phase composed principally of columbium and tantalum carbides with a minor amount of chromium, and a eutectic phase, significantly affecting strength, composed principally of chromium-tungsten carbide and solid solution.

The carbides present, within the range of 0.83-0.97 percent provide an extremely stable grain size for the wrought material, optimally in the range of ASTM 45, to the solidus temperature of the alloy. The carbides are thus stable to very high temperatures and do not readily go into solution up to these temperatures. In wrought form, a carbon content of 0.9 percent, in combination with the specified carbide formers, creates the optimum high temperature strength. If the carbide content is lower, the resulting alloy exhibits poor initial creep properties while a higher percentage of carbides hinders workability. For example, a carbon content of 1.3 percent combined with a high percentage of carbide formers, created an alloy which proved diiiicult to process.

Chromium, titanium and the refractory metals are, of course, carbide formers. In addition, chromium is responsible for imparting corrosion resistance to the alloy.

The titanium and zirconium constituents similarly appear to signiiicantly atect the alloy properties. In an alloy of a chemistry very similar to the WS-6 chemistry except a titanium content of 1.8 percent and a zirconium content of 0.9 percent, alloy performance was decreased. Similarly, with an alloy containing 1.5 percent tantalum, 0.5 percent columbium, 1.3 percent carbon, 1.2 percent titaniu-m and 0.6 percent zirconium, but otherwise similar to the WS-6 alloy, properties were vastly inferior-0.5% creep, 0.1-4 hours, as compared to 10-50 hours with WS-6. An alloy with high titanium (1.8%) and high columbium (1.3%) at a carbon level of 0.9% was found hard to work.

Iron, while not essential in all cases, is preferably added to stabilize the alloy with respect to its allotropic transformation and to provide the desired face centered cubic structure.

The refractory metals, tungsten, tantalum and columbium are, of course, vigorous carbide-farmers, as previously mentioned. While the tantalum and columbium constituents appear primarily in combination with the carbon, the tungsten appears both combined with carbon and as a solid solution strengthener. At tlmgsteu levels below that specified herein strength and apparently all-oy stability will be reduced. It is felt that at the preferred chemistry about the maximum tungsten, from a solubility standpoint, is provided in the matrix and that this contributes to the relative insolubility of the carbides which, of course, in turn provide grain size stability.

With respect to the tantalum and columbium, it has been observed that an alloy, basically to the VVS-6 chemistry but containing 5 percent tantalum and 1 percent c0- lumbium failed in rupture at hours whereas at the WS-6 chemistry up to S52 hours have been achieved. Both tantalum and columbium are, however, present in the alloy principally as carbides.

In the preferred speciication chemistry nickel is present in the alloy to the extent of about 14-16 weight percent. lt is provided primarily to furnish ductility to the mate rial and, thus, it appears primarily in solid solution.

It is expected that the addition of the ingredients frequently added to promote oxide adherence, such as yttriuni and the rare earth metals such as lanthanum may be made if limited in amount. The alloy, however, is not tolerant of large amounts of lanthanum since it has an adverse aiiect on alloy ductility. An alloy to the WS-6 chemistry containing in addition 0.2 percent lanthanum, for example, cracked on rolling.

The density of the WS-6 alloy is 0.316 lb./in.3 compared to a density of 0.329 lb./in.3 and 0.330 1b./in.3 for Alloys A and B, respectively. It has an incipient melting temperature of 23502360 F. Hardness as extruded is 39.5 Rc and after 2325 F./ 16 hr./anneal 20.5 Rc. It is readily welded with either WS6 or Alloy B although Alloy B is preferred for maximum creep capability.

As may be seen by reference to the following data and FIGS. 4-6 the wrought alloys of the present invention provide: rupture capabilities to over 2100 F.; an 80- l00 F. strength improvement over Alloy B at 1600" F.; an approximate 200 F. strength improvement over Alloy C with hot corrosion resistance equivalent thereto; lower density than the better contemporary cobalt-base sheet alloys; and excellent weldability.

The alloy is, in addition, thermally stable. Recrystallization does not occur below 2275 F. and signiicant grain growth occurs only after extended exposure to temperatures near the solidus (2350u F.), with grain growth inhibited by the presence of primary carbides at the grain boundaries. The structure in the as-cast, as rolled and recrystallized conditions are shown in FIGS. l, 2 and 3, respectively.

The creep rupture properties and tensile strength of the alloy are shown in the following tables.

TABLE L CREEP RUPTURE PROPERTIES oF Ws- SHEET (2,325 F.-0.5 hr. anneal) Hours to- Elongatien Sheet Teml., Stress, size p.s.1. 0.5% 1.0% Rupture Prior Final 060 1,600 20, 000 6 18 140. 5 8.0 9. 9 050" 1, 800 8, 000 21 50 193. 5 7. 2 8. 8 060Il 2, 000 4, 000 3. 5 8 47. 9 12.1 O60 2, 200 1, 500 l. 2 32. 3 24. 4

TABLE IL TENSILE PROPERTIES OF WS- SHEET In addition to the evaluation of material as conventionally cast, extruded and rolled, suitable billets have been produced and extruded from alloy powder. Utilizing powder metallurgy techniques high volume fractions of spherical type carbides can be provided in the cobalt alloy matrix. In general, the WS-6 sheet has been annealed at 2250 F. for 4 hours prior to fabrication, and testing has revealed that the optimum stress rupture heat treatment for the WS-6 0.040 inch sheet is 2325 F./ 0.5 hour.

The invention in its broader aspects is not limited to the specic steps, methods, compositions, combinations and improvements described for the purposes of illustration, but departures may be made therefrom within the scope of the appended claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A wrought cobalt-base alloy for high temperature use which consists essentially of, by weight, 0.83-097 percent carbon, 24-26 percent chromium, 0.8-1 percent titanium, up to about 1.2 percent iron, about 14-16 percent nickel, 7.5-8 percent tungsten, 0.350.45 percent zirconium, about 0.8-1.2 percent columbium, about 2.5-3 percent tantalum, balance substantially cobalt.

Z. An alloy according to claim 1 containing, by weight, 0.8-1.2 percent iron.

3. An alloy sheet having a thickness not exceeding about 0.1 inch thickness at a nominal chemistry consisting essentially of, by weight, 0.9 percent carbon, 25 percent chromium, 0.9 percent titanium, 1 percent iron, 15 percent nickel, 7.7 percent tungsten, 0.4 percent zirconium, 1 percent columbium, 2.7 percent tantalum, balance cobalt.

RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 148-32 ggg@ UNITED STATES PATENT OFFICE CERTIFICATE @F CORREC'EIN Patent No. 3,667,939 Dated June 6, 1972 Inventor(s) J. Hecht; Richard J. Fenton I-t is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The Filing Date of December 30, 1971 listed througloui:

the patent should be corrected tov read December 30, 1970 Signed and sealed this 3rd -day of" Octoberl 1972.

(Sbix-TL) Attest: v A

EDWARD MQFLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

